Conventionally, in a mobile communication system such as a PHS (Personal Handyphone System), communication is established between a mobile terminal apparatus (hereinafter referred to as a terminal or PS (Personal Station)) and a wireless base station (hereinafter referred to as a base station or a CS (Cell Station)), using a known π/4 shift QPSK (Quadrature Phase Shift Keying) modulation method.
FIG. 8A shows an arrangement of symbol points in accordance with the π/4 shift QPSK modulation method on an IQ coordinate plane. Referring to FIG. 8A, more specifically, in the π/4 shift QPSK modulation method, a symbol point of a received signal corresponds to any of four signal points positioned concentrically on the IP coordinate plane, as is well known, and therefore, 2-bits of data representing any of the four signal points can be transmitted at one time.
Conventionally, both in the stage of establishing wireless communication between the terminal and the base station through a control channel CCH (Control Channel) and in the stage of performing desired data communication of voice or the like through a traffic channel TCH (Traffic Channel), a fixed modulation method, such as the π/4 shift QPSK modulation method mentioned above, is used for communication.
In recent mobile communication system, data transmission of larger amount at higher speed than the conventional voice communication is required, such as in the case of data communication. Therefore, methods of modulation having larger multi-value number than the conventional π/4 shift QPSK modulation method have been developed.
As an example of such modulation method of larger multi-value number, 16QAM (Quadrature Amplitude Modulation) method has been known and practically used in some type of data communications.
FIG. 8B shows an arrangement of symbol points in accordance with the 16QAM modulation method on the IQ coordinate plane. Referring to FIG. 8B, according to the 16QAM method, a symbol point of a received signal corresponds to any of a total of 16 signal points on the entire plane, with the signal points being arranged in a lattice form of four points on each quadrant, on the IQ coordinate plane. Therefore, it is possible to transmit 4-bits of data representing any of the 16 signal points, at one time.
When a modulation method having larger multi-value number such as the 16QAM method is employed as the modulation method of the PHS, symbol points may possibly be recognized erroneously if communication environment of the transmission path is unsatisfactory (when the transmission path has considerable noise/interfering waves). Namely, this method has a characteristic that it has higher communication rate and is more prone to reception error, than the π/4 shift QPSK modulation method shown in FIG. 8A.
Generally, in the stage of establishing wireless connection through CCH, routine information only is transmitted between the terminal and the base station, and in this stage, higher rate of communication is not required. Therefore, communication rate attained by the conventional π/4 shift QPSK modulation method is sufficient.
In data communication through TCH, however, higher rate of data communication has been strongly desired, in order to transmit large amount of data.
In view of the foregoing, an idea of adaptive modulation has been proposed, in which communication is done in the conventional π/4 shift QPSK modulation method in the stage of establishing wireless connection through CCH, and in the stage of data communication through TCH after the connection is established, the modulation method is switched to the 16QAM method.
In the stage of establishing connection between the terminal and the base station, by way of example, the terminal transmits a wireless connection request to the base station. The base station measures, as a parameter representing a state of communication environment of the transmission path, a known RSSI (Received Signal Strength Indication) value as a D (Desired) wave level, based on a received signal power level of the desired signal, determines whether the measured D wave level exceeds a threshold value to realize communication with stable communication quality under the π/4 shift QPSK modulation method, and determines whether a wireless channel should be allocated or not, dependent on the result of determination.
The D wave level (RSSI value) corresponds to S of the signal-to-noise ratio (S/N ratio), and therefore, when the noise N received by the base station is known beforehand, the D wave level may be considered equivalent to the S/N ratio.
More specifically, the threshold value is set to such a D wave level that satisfies a error rate BER (Bit Error Rate) that can realize stable communication quality when communication is done in accordance with the π/4 shift QPSK modulation method. When the actually measured D wave level does not reach the threshold value, communication quality degrades, possibly resulting in reception error or disruption of wireless connection, and hence normal and stable communication is impossible.
Therefore, only when the actually measured D wave level exceeds the threshold value, a wireless channel is allocated (that is, connection is permitted) to the terminal requesting connection, and when the level does not reach the threshold value, wireless channel is not allocated (connection is rejected).
FIG. 9 is an illustration showing, with time, a communication procedure between a terminal (PS) and a base station (CS), when the modulation method is switched as described above (adaptive modulation).
First, in the stage of establishing wireless connection, communication takes place in accordance with the π/4 shift QPSK modulation method. A connection request may be made either by the terminal or the base station. In this example, it is assumed that the terminal transmitted the request.
First, through a link channel establishing phase, signals related to the wireless connection request are exchanged between the terminal and the base station. Specifically, the D wave level from the terminal is measured on the side of the base station, whether the measured D wave level exceeds the threshold value for allocating wireless channel of the π/4 shift QPSK modulation method described above or not is determined, and if the level exceeds the threshold value, subsequent process for establishing wireless connection is executed.
Specifically, exchange for a service channel establishing phase takes place, and when the terminal and the base station are synchronized thereby, message control (call control and the like) is executed between the terminal and the base station. Operations thus far correspond to the stage of establishing wireless connection executed by communication in accordance with the π/4 shift QPSK modulation method.
Thereafter, to enter the stage of data communication, assume that the modulation method is switched from the π/4 shift QPSK modulation method to the 16QAM method, in order to attain higher communication rate.
In this case, there is a possibility that communication quality degrades, causing data communication failure, as represented by dotted lines in FIG. 9, because of the following reasons.
Different modulation methods have different, unique error rate characteristics, as will be described later, and by way of example, the π/4 shift QPSK modulation method and the 16QAM method have much different characteristics. Therefore, even if the D wave level of a signal from the terminal at the base station satisfies the channel allocation threshold value of the π/4 shift QPSK modulation method, stable communication quality is not always ensured under the 16QAM modulation method.
Specifically, even when normal and stable communication is possible by the π/4 shift QPSK modulation method, wireless communication quality degrades when the method is switched to the 16QAM modulation method, and normal communication possibly fails because of communication error or an accident such as disruption of the wireless connection.
This point will be described in detail with reference to FIG. 10. FIG. 10 is a graph representing relation between the communication environment of the transmission path and the error rate in the received signal, for the π/4 shift QPSK modulation method and the 16QAM modulation method. It is noted that FIG. 10 is an exemplary graph allowing visual recognition of error rate with respect to the modulation method, and specific values are not necessarily precise.
Specifically, the abscissa of FIG. 10 represents signal-to-noise ratio (S/N ratio: considered as equivalent to D wave level) on the transmission path, and the ordinate represents error rate BER of the received signal.
Similar characteristics can be seen when the abscissa is replaced by a ratio of the D wave level on the transmission path to undesired signal level, that is, U (Undesired) wave level (D/U ratio).
Generally, in the digital communication system, a signal waveform received on the receiving side is returned to the digital information that was intended by the transmission side, by a demodulating process. The digital information is binary information of “0” or “1”, and therefore, it is basically noise-free.
However, as described with reference to FIGS. 8A and 8B, when a large noise occurs in the middle of the transmission path, digital information of “0” or “1” to be transmitted may be transmitted erroneously because of the noise.
Transmission error is caused by noise or interfering wave as mentioned above, and from the S/N ratio (or D/U ratio) of the noise introduced to the transmission path and the modulated wave (desired transmission signal wave), an error rate (BER), which indicates how frequently error occurs when a large amount of information is transmitted, can roughly be assessed.
Specifically, the error rate is closely related to the S/N ratio (D/U ratio) on the transmission path, and hence it can be derived by calculation using well-known statistical theory: It is also known that different modulation method results in different error rate, even if the S/N ratio (D/U ratio) on the transmission path is the same.
Returning to the characteristic diagram of FIG. 10, the relation between the S/N ratio and the error rate BER of the π/4 shift QPSK modulation method is plotted in dotted line, while the relation between the S/N ratio and the error rate BER of the 16QAM method is plotted in chain-dotted line.
Referring to the example of FIG. 10, assume that the error rate BER of, for example, 10−4 is to be ensured regardless of the modulation method in order to maintain stable communication quality. From the graph of FIG. 10, it can be seen that by the π/4 shift QPSK modulation method (dotted line), the error rate can be suppressed to 10−4 or lower with the S/N ratio of about 6 dB or higher, whereas by the 16QAM method, the S/N ratio must be at least about 11 dB in order to suppress the error rate to 10−4 or lower.
Such difference in error rate characteristics among various modulation methods may lead to such a situation that, the S/N ratio of the transmission path exceeds the threshold value of S/N ratio for stable communication under the π/4 shift QPSK modulation method (about 6 dB in the example of FIGS. 8A and 8B above) in the stage of establishing wireless connection and a wireless channel is allocated in the link channel establishing phase, while the S/N ratio at this time is below the S/N ratio for stable communication under the 16QAM modulation method (in the example of FIG. 10, about 11 dB or higher): if the modulation method is switched in this state from the π/4 shift QPSK modulation method to the 16QAM method, the error rate BER degrades (in the example of FIG. 10 above, the error rate becomes higher than 10−4) and wireless communication quality degrades, possibly causing communication error or disruption of wireless connection.
The same applies when D/U ratio is referred to in place of the S/N ratio of the transmission path.
As described above, a conventional wireless apparatus that supports adaptive modulation has a problem that even when wireless connection is once established under a modulation method of smaller multi-value number and a channel is allocated, normal communication may fail due to degraded communication quality when the modulation method is switched to one having larger multi-value number during communication.